The "boot" you refer to is called a Fernco coupler and they come in various sizes and configurations. They are used exactly for applications such as the one you are doing. They not only allow you to mate pipes of different materials, they are flexible enough to allow a slight correction in alignment. They are as dependable as they pipe. As far as cutting cast iron, that is a bit more of a challenge. There are snap cutters that can be rented, but most guys use a recipcical saw with a metal cutting blade, or you can use a hand grinder (this is messy and can be dangerous especially with a 7" or larger tool) From the picture I get of what you're wanting to do, I think you'll need 2 Ferncos, but they are cheap and easy to use.